Thirty to forty percent of the human population under age 40 develop an ocular refractive error requiring correction by glasses, contact lenses, or surgical means. Refractive errors result when the optical elements of the eye, the cornea and the lens, fail to image light directly on the retina. If the image is focused in front of the retina, myopia (nearsightedness) exists. If the eye image is focused behind the retina, hyperopia (farsightedness) exists. The focusing power of the eye is measured in units called diopters.
Approximately 20% of the patients under 40 having vision defects cannot wear contact lenses because the contact lenses do not fit (become dislodged and/or are very uncomfortable), or they fail to provide the requisite optical correction, or both. In addition, many patients who currently wear contact lenses are not satisfied with the length of time they can wear their lenses or with the visual acuity their contact lenses provide.
Over age 40, the percentage of the population requiring vision correction dramatically increases and the problems encountered with existing contact lenses become much more common and acute.
Standard contact lenses are rotationally symmetrical and spherical. The human cornea, however, is an "asymmetrically aspheric" surface.
"Aspheric" on the one hand means that the radius of curvature along a corneal "meridian" (which is an imaginary line on the corneal surface passing through the geometric center of the cornea, analogous to a geographic meridian) is not a constant. Indeed, the corneal curvature flattens progressively from the geometric center to the periphery. "Asymmetric" on the other hand means that the profile of the corneal curvature along a half-meridian is not the same as (i.e., it is not a mirror image of) the other half of the same meridian. The degree to which corneas are aspheric and/or asymmetrical varies from patient to patient.
Standard spherical lenses do not match the corneal curvature and geometry, and therefore do not fit properly. The more irregular the patient's cornea the worse the fit, such that about 20% of the patients under age 40 are unable to wear standard contact lenses.
In an effort to alleviate these problems, manufacturers developed lenses with varying curvatures on their posterior surface. For example, U.S. Pat. No. 5,114,628 discloses aspherical contact lenses made using corneal topographic data to control a lathe. (The data provide information on the slope of the corneal surface at different points on the cornea but are based on measurements in two dimensions interpreted three-dimensionally.) The resultant lens is aspherical (in both the anterior and posterior surface) but inherently symmetrical. Such a lens may fit some patients better than the standard spherical lenses. But other patients may experience more discomfort than with the spherical lenses. Thus, this type of aspherical symmetric lens does not provide a substantial improvement in the number of patients that can comfortably wear contact lenses and/or wear contact lenses that provide them with the requisite visual acuity.
Other U.S. patents (e.g., U.S. Pat. No. 4,923,467, U.S. Pat. Nos. 5,104,408 and 5,156,622)disclose the shaping of a lenticule which is implanted within the substance of the cornea. These lenticules are not contact lenses. The lenticules described in these patents are shaped based on corneal topographic mapping data. A laser is used to ablate material from a lens blank. However, the fit problems encountered with these implants are not the same as those encountered with contact lenses. For example, unlike a contact lens, implanted lenticulas are stationary and once installed in the corneal stroma they neither "rock" on the cornea nor float on a tear film and are not subject to external forces such as eyelid pressure or the force of gravity.
U.S. Pat. No. 2,264,080 to Hunter discloses a system for manufacturing a "contoured" scleral contact lens, i.e., a lens resting on the sclera, not on the cornea. Hunter teaches the creation of a mold of the surface of the eye which is then used as a "template" to mechanically guide a grinder over the surface of a lens blank. The grinder receives information about the meridional topography of the mold and travels over the surface of the lens blank in a back-and-forth fashion along meridians of the lens. Hunter's scleral lens intentionally has sufficient clearance from the cornea to avoid any contact with the surface of the cornea. Moreover, his method of manufacture causes "ridges" to be formed on the posterior surface of the lens, which if present on a contact lens closely fitted to the cornea would cause discomfort to the wearer.
The need in the art for better fitting contact lenses is illustrated in an article in Ophthalmology Times Nov. 1, 1992, p. 82, which discloses that future areas of research will involve increasingly sophisticated aspheric optics and refinements on contact lenses based on the asphericity of the cornea.
In other words, although both the asphericity and asymmetry of the cornea may have been recognized in the art, only the asphericity of the cornea has been taken into account in contact lens design. The present inventor unexpectedly discovered that if part of the contact lens accurately mimics the surface of the cornea in both asphericity and asymmetry, a better fit and/or a better vision correction can be achieved consistently. Thus, there is a need in the art for a contact lens that will decrease or eliminate the number of patients of all ages who currently cannot wear contact lenses, and provide better comfort and/or visual acuity (including better correction of astigmatism) for patients who now wear contact lenses.